Although manufacturing growth slowed, May marked the 34th month in a row that activity expanded.

The index came in just below the expectations of economists surveyed by Briefing.com, who forecast the index would fall to 54.

The overall decline was driven by a dip in production, which fell by 5.4 points in May. The employment reading also came in lower, slipping by 0.4 point.

"It was a little bit of a disappointment to see a slowdown in growth, but it's growth nonetheless," said Sarah Watt, an economic analyst at Wells Fargo."There's a lot of uncertainty in the air with the debt crisis in Europe and policy here in the U.S. that's making companies reticent to invest."

Slow growth in manufacturing was reflected in another report, the government's May jobs data. The Labor Department reported that 12,000 manufacturing jobs were added in May, which was up from 9,000 in April, but below the 42,000 jobs added in the sector in April.